AWI researchers develop new method to detect platelet ice over large distances
Sea ice physicists of the Alfred Wegener Institute (AWI) have developed a new method that allows them for the first time to efficiently determine the distribution and thickness of what researchers call a sub-ice platelet layer.
This several metre thick layer of delicate ice crystals is predominantly found beneath coastal Antarctic sea ice, and at present knowledge about its spatial distribution is very limited. This phenomenon, which is also known as platelet ice, is of central importance in the coastal regions of the Antarctic, influencing sea ice properties and the associated ecosystem in various ways, and serving as an indicator for the state of melting ice shelves. The researchers published their results with open access in the current issue of the journal Geophysical Research Letters.
Every winter in the Southern Hemisphere, the ocean around the Antarctic continent freezes. The “normal” sea ice formed on the surface of the Southern Ocean, however, is not the only ice that forms in the sea. During the same period, a remarkable habitat develops hidden beneath the solid sea ice cover: a several metre thick layer of loose ice crystals. Some areas underneath sea ice in coastal Antarctica then resemble a cocktail glass filled with crushed ice – the difference being that the crystals in this layer grow to disc-shaped, millimetre-thin platelets.
This phenomenon was discovered by researchers more than a century ago, but for a long time, little was known about this peculiar ice type. In the past decades, researchers have discovered that platelet ice plays a significant role in the sea ice mass balance in some regions around the Antarctic, and that it represents a unique and productive habitat. Countless algae thrive on the platelets, which are food for a myriad of small crustaceans and fish that seek shelter between the platelets.
Knowledge about the thickness and extent of platelet ice is still very limited, because of its “hidden” nature. Most of the time, researchers only came across it by chance – for example, while drilling through the sea ice to measure its thickness. A team of researchers of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), the Jacobs University Bremen and Uppsala University have now succeeded to develop an efficient method to determine the distribution, thickness and volume of platelet ice over large distances.
As the scientists report in a recent issue of the journal Geophysical Research Letters, they used a so-called multifrequency EM device to determine the platelet ice volume. EM is short for "electromagnetic induction sounding", a geophysical method sensitive to the electrical conductivities within the subsurface.
The electrical conductivity of solid sea ice, for instance, differs significantly from that of the salty seawater underneath. This means that, using the EM device, scientists are able to identify the transition from sea ice to seawater, thus allowing them to calculate the ice thickness.
However, the transition between the solid sea ice and the loose platelet layer is much less pronounced. Conventional EM methods that only use a single frequency are unable to distinguish the platelet layer from sea ice or seawater. Using several different frequencies, scientists are now able to reliably determine these transitions without having to drill holes into the ice and measuring the thicknesses using measuring tapes.
"We were really surprised by our own results," says AWI sea ice physicist and co-author Dr Mario Hoppmann. "With our new approach, we weren’t only just able to determine the thickness of the platelet layer. We were even able to calculate the fraction of ice within this layer by subtracting the volume of sea water in between the platelets."
In order to collect as much data as possible, the researchers conducted several surveys across large parts of the frozen Atka Bay. The bay is located in the Weddell Sea, near the German Antarctic research station Neumayer III. The researchers placed the multifrequency EM instrument in a kayak, which in turn was attached to a snowmobile. They moved the tandem across the sea ice of Atka Bay for many days and several hours at a time.
"One of the things we noticed was that the evolution of the platelet layer has an annual rhythm," says Mario Hoppmann. In June, at the beginning of the Antarctic winter, platelets begin to accumulate under the sea ice. Over the course of the winter, the layer grows; by the end of the winter, in December, it is several meters thick, after which it shrinks again during the summer.
The researchers are convinced that platelet ice plays an important role in the ice regime of the Antarctic. After all, the seasonal sea ice in Atka Bay freezes to an average thickness of two metres in the winter. The platelet layer underneath, however, reaches an average thickness of five metres over the course of a year. In some places it was up to ten metres thick. This means that a significant amount of the ice exists in the form of platelets. "To understand the situation of the Antarctic sea ice and to assess a possible influence of climate change, it is likely that more account must be taken of platelet ice," says Mario Hoppmann.
It is not currently possible to properly assess the significance of the platelet ice across the Antarctic. The new findings give cause for hope that its distribution and therefore also its role will soon be understood to the same extent as its formation.
Platelets, which later accumulate in the platelet layer, form beneath the ice shelves of the Antarctic, those parts of the mighty ice sheet that float on the sea. The platelet ice cycle begins as salt-rich water in the coastal ocean sinks and slides underneath the ice shelves, which it then slowly melts. The result: The melted fresh water mixes with the salty ocean water underneath the ice shelves. On the surface of the sea, this water mix would freeze immediately, because its temperature is well below the surface freezing point. Because of the high water pressure in the depth of the sea, the mix initially stays liquid – physicists call this a "potentially supercooled" state.
Because this water mass has a lower density than the surrounding seawater, it slowly rises at the base of the ice shelves. The water pressure decreases and as soon as a critical shallower water depth is reached, tiny little ice crystals start to form. These then grow to form those delicate ice platelets that later accumulate as platelet ice underneath the sea ice at the surface.
Notes for Editors:
The study was published in open access format under the following title in the journal Geophysical Research Letters: P. A. Hunkeler, M. Hoppmann, S. Hendricks, T. Kalscheuer & R. Gerdes, 2015: A glimpse beneath Antarctic sea ice: Platelet layer volume from multifrequency electromagnetic induction sounding. Geophysical Research Letters. DOI: 10.1002/2015GL065074; Link: http://onlinelibrary.wiley.com/doi/10.1002/2015GL065074/full
Printable photographs and video material can be found in the online version of this press release at: http://www.awi.de/nc/en/about-us/service/press.html
Your scientific contact at the Alfred Wegener Institute in Bremerhaven is Dr Mario Hoppmann, phone +49 (0)471 4831-2907 (e-mail: Mario.Hoppmann(at)awi.de).
Your contact in the Communications and Media Department is Sina Löschke, phone +49 (0)471 4831-2008 (e-mail: medien(at)awi.de).
The Alfred Wegener Institute researches in the Arctic, the Antarctic and oceans in the central and high latitudes. It coordinates polar research in Germany and provides important infrastructure such as the research icebreaker Polarstern and stations in the Arctic and Antarctic for the international science community. The Alfred Wegener Institute is one of the 18 research centres belonging to the Helmholtz Association, which is Germany's largest scientific organisation.
Ralf Röchert | idw - Informationsdienst Wissenschaft
Mountain glaciers shrinking across the West
23.10.2017 | University of Washington
Climate change weakens Walker circulation
20.10.2017 | MARUM - Zentrum für Marine Umweltwissenschaften an der Universität Bremen
University of Maryland researchers contribute to historic detection of gravitational waves and light created by event
On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...
Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.
Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....
Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).
When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...
Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.
How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...
Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.
It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...
23.10.2017 | Event News
17.10.2017 | Event News
10.10.2017 | Event News
23.10.2017 | Physics and Astronomy
23.10.2017 | Earth Sciences
23.10.2017 | Health and Medicine